Fusobacterium spp. are gram-negative, obligately anaerobic and pleomorphically rod shaped bacterium responsible for a variety of necrotic infections in animals and in humans (Langworth, Bacteriol. Rev., 41, 373-390 (1977)). Fusobacterium necrophorum is classified into two subspecies: F. necrophorum subsp. necrophorum and F. necrophorum subsp. funduliforme and are responsible for a number of clinical manifestations in various species of animals, such as cattle, sheep, and swine to include; hepatic abscesses, foot rot, laminitis, purulent and interdigital dermatitis, contagious eethyma, necrotic rhinitis, and necrotic laryngitis.
In humans, F. necrophorum and F. nucleatum are considered to be the most pathogenic and are the causative agent of skin ulcers, peritonsillar abscesses, septic arthritis, Lemierre's syndrome, periodontal diseases and endocarditis. A number of other species of Fusobacterium have been implicated as the etiological agent in a variety of diseases, for example, F. ulcercans (skin ulcers), F. russi (animal bite infections), and F. varium (eye infections) (Smith et al., Epidemiol Infect., 110, 499-506 (1993)).
The bacterium produces a number of virulence factors that are responsible for the pathogenesis of the organism, including a potent secreted leukotoxin which has been shown to be specifically toxic to ruminant polymorphonuclear leukocytes (Tan et al., Vet. Res. Commun. 20, 113-140 (1996)). The importance of leukotoxin as an important virulence factor has been well documented. For instance, experiments have indicated a correlation between toxin production and the ability of F. necrophorum to induce abscesses in laboratory animals (Coyle et al., Am. J. Vet. Res., 40, 274-276. (1979), and Tan et al., Am. J. Vet. Res., 55, 515. (1994)). Experiments have also shown that non-leukotoxin producing strains are unable to induce foot abscesses in cattle following challenge. It has also been shown that neutralizing antibody produced by an inactivated toxoid derived from leukotoxin reduced infection and liver abscesses in vaccinated cattle.
A number of commercial killed whole cell bacterins have been used to control necrotic infection in farm animals incorporating multiple strains including the most prevalent serotypes such as biotype A (F. necrophorum subsp. necrophorum). Another approach to vaccine development has been the incorporation of leukotoxin as a toxoid to prevent the pathological effect of the secreted toxin (Saginala et al., J. Anim. Sci., 75, 11601166 (1997)). While conventional vaccines have shown some degree of efficacy in preventing colonization and infection with F. necrophorum, adequate protection in cattle is still lacking.
Divalent metal ions such as iron, cobalt, copper, magnesium, manganese, molybdenum, nickel, selenium, and zinc and are trace elements often required for the survival of bacteria infecting both animal and human hosts. These trace metal elements are used by bacteria as cofactors for enzymes that catalyze biochemical reactions for various metabolic pathways required by the organism. The impact of iron on the pathogenesis of bacteria has been studied extensively. Iron is essential for nearly all life and is required for enzymatic and metabolic pathways of organisms at all phylogenic levels.
The ability of Fusobacterium to evade the natural defense mechanisms of the vertebrate host depends in part on its ability to obtain host iron, which in turn, directly influences the host-pathogen interaction. Because of iron's essential nature, vertebrate hosts have developed elaborate mechanisms to bind iron in body fluids (e.g., transferrin in blood and lymph fluids and lactoferrin in external secretions). These high affinity iron binding proteins create an iron restricted environment within the host, reducing the level of iron to approximately 10−18 molar, a concentration too low to support the growth of nearly all bacteria. These iron sequestering mechanisms of the host act as a natural defense mechanism to combat bacterial invasion. To circumvent these iron-restrictive conditions many bacterial species have evolved mechanisms for obtaining iron. The most common mechanisms include the diffusion of soluble iron through porins and specialized transport systems that mediate the uptake of iron by siderophores. This latter system is one of the most common and well-studied mechanisms for iron acquisition and involves the specific chelation of ferric iron by siderophores and the synthesis of their cognate transport systems, which permits the bacteria to continue to replicate and overcome the non-specific defense mechanisms of the host. Continued replication, and thus each step in the infectious process, is ultimately dependent on the ability of the organism to obtain iron from its host.
With so many basic functions relying on the availability of iron, bacteria have evolved a complex regulatory network for acquiring iron under varying physiological conditions. Under anaerobic conditions, iron is present in the soluble ferrous form (Fe II) and can freely diffuse through outer membrane porins into the periplasm. For instance, in E. coli the FeoAB transport system present in the cytoplasmic membrane will transport the ferrous iron molecules into the cell cytoplasm. Under aerobic conditions and neutral pH, iron is primarily present in the insoluble ferric form (Fe III) and cannot pass through the outer membrane porins by passive diffusion. Instead, molecules called siderophores are secreted by bacteria, which have a high affinity for ferric iron. The ferric-siderophore complexes are recognized by receptors in the outer membrane, collectively referred to as the TonB-dependent receptors. These receptors, once bound to loaded siderophores, are believed to interact with TonB and its associated proteins localized in the periplasm and cytoplasmic membrane. These protein-protein interactions, though poorly understood, serve to provide the energy necessary to transport the ferri-siderophore complexes across the outer membrane and through the periplasmic space. ABC transport systems present in the cytoplasmic membrane serve to transport the iron-siderophore complexes across the cytoplasmic membrane. Reductase enzymes reduce the ferric iron to its ferrous form, which dissociates it from the siderophore and releases iron into the cell.
Several species of pathogenic bacteria use additional mechanisms to obtain iron from mammalian hosts, including the direct binding of transferrin, heme, and other heme-containing compounds. The receptor proteins that bind these iron-containing molecules most likely rely on the TonB complex for the energy required to transport heme across the outer membrane, similar to the iron-siderophore complexes. Specialized ABC transporters are then used to transport the heme across the cytoplasmic membrane. In addition, some bacteria secrete hemophores, small molecules that can bind heme and present it to receptors on the bacterial cell surface. Several pathogenic species also produce hemolysins, which are toxins that lyse red blood cells, releasing heme and hemoglobin for uptake by the bacteria.
The outer membrane proteins of gram-negative bacteria control the selective permeability of many essential nutrients critical to the survival of bacteria, including all pathogenic bacteria that cause disease in animals and man. This selective permeability of nutrients is controlled by a class of membrane proteins called porins. It now appears that the majority of the outer membrane proteins on the surface of gram-negative bacteria are porins, identified as the general porins (e.g., OmpF), monomeric porins (e.g., OmpA), the specific porins (e.g., the maltose-specific porin LamB) and the TonB-dependent, gated porins (e.g., the siderophore receptor FepA). The porin class of proteins generally share structural features, including the presence of beta-barrels that span the outer membrane.
Little is known regarding the iron-acquisition by Fusobacterium spp, and genomic comparisons are difficult since the genome of only one strain of Fusobacterium nucleatum have been completely sequenced, F. nucleatum strain ATCC 25586 (Kapatral et al., J. Bacteriol., 184, 2005-2018 (2002)). However, this genomic sequence was recently used in a comparison with a partially sequenced genome of F. nucleatum subspp. vincentii (Kapatral et al., Genome Res., 13, 1180-1189 (2003)) to investigate differences among these two subspecies. The results suggested that there were differences between the two genomes with respect to the iron uptake systems. Although iron transport systems were discovered in both genomes, the genome of strain ATCC 25586 contains three additional iron-specific ABC transport systems. In addition, hemin receptor proteins appear to be encoded by both genomes, but while the subspp. vincentii isolate encodes three receptors, the genome of strain ATCC 25586 apparently encodes five such proteins. Furthermore, the feoAB genes, encoding a putative ferrous iron transport system, are only found in the genome of the subspp. vincentii isolate. Since both organisms are obligate anaerobes and ferrous iron is the predominant form of the metal under anaerobic conditions, strain ATCC 25586 may have a second mechanism for uptake of ferrous iron. Given the differences among these two subspecies of F. nucleatum, it is likely that there will be many differences among the iron uptake systems between other Fusobacterium species. Therefore, the F. nucleatum genomic data may not be useful for predicting the presence or absence of iron acquisition systems in other species of Fusobacterium. 